The present invention relates to an optical head for recording and reading information used for a compact disc (CD) device, a digital versatile disc (DVD) device and a magneto-optical disc device (MO) and to an objective lens used in the optical head, and in particular, to those capable of being applied to the aforementioned devices wherein a light source is a blue laser.
The optical head has an objective lens which forms an image of a laser beam on an information recording surface of an optical disk such as CD, DVD and MO as a spot and has a driving means which drives the objective lens for tracking and focusing.
In JP-A No. 5-210860, it is proposed that a position of the center of gravity of an optical head is made to agree positionally with a position of a driving point for the optical head for preventing resonance of the optical head caused in the course of tracking or focusing.
Further, in the objective lens, chromatic aberration is caused by wavelength changes resulted from output changes or temperature changes of a semiconductor laser representing a light source or is caused by changes of refractive index resulted from lens temperatures, however, the chromatic aberration cannot be corrected by the aforesaid one or two convex lenses. Therefore, correction of chromatic aberration conducted by the use of a diffracting lens is proposed in, for example, JP-A No. 6-82725, JP-A No. 6-242373, JP-A No. 7-294707, JP-A No. 11-95145, JP-A No. 11-337818, JP-A No. 2000-81566 and JP-A No. 2002-298422.
Since the objective lens having optical elements for correcting chromatic aberration is composed of a plurality of optical elements, the center of gravity thereof is in a position which is different from that of the center of gravity of the objective lens composed of one optical element. Therefore, if an objective lens having plural optical elements is mounted on a supporting frame of the optical head in the same way as in the conventional objective lens having one optical element, it results in that the objective lens is supported at the position deviated from the center of gravity of the objective lens.
Though the center of gravity of a movable portion in an optical head including an objective lens is made to agree positionally with a driving point, in JP-A No. 5-210860, there still is a problem that movements of the movable portion become complicated to cause a tilt and prevention of vibration becomes difficult, even when the center of gravity of the entire movable portion is made to agree with its driving point, if the center of gravity of the objective lens is deviated from its supporting point, which has been a problem.
In FIG. 12 of JP-A No. 5-210860, there is described an optical head wherein an objective lens is arranged at the position that is away from the driving point of a movable portion including an objective lens, and in FIG. 14, there is described an optical head wherein an objective lens is arranged on a line connecting driving points substantially. However, there is a possibility that the problems mentioned above are still caused on both optical heads described in FIG. 12 and FIG. 14 in JP-A No. 5-210860.